I was thinking like 3/4 to 1/2 inch tubes of solid plastic. So maybe heavy-duty bolt cutters would be a good idea to get them off.

The crux of the idea is that only way to get the cuffs off is to break the cuff with a specific brute force that requires a large unconcealable device. Even with the device, you still need time/space to use it (5-10 mins).

So pretend that we would need a large UV lamp and 5 mins to melt through.